Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system such as a copying machine, a printer, or a multifunction peripheral (MFP).
Description of the Related Art
An image forming apparatus using an electrophotographic system, an electrostatic recording system, or the like charges the surface of an image carrier such as a photosensitive drum to a predetermined potential. An electrostatic latent image is formed on the charged surface of the image carrier, and this electrostatic latent image is developed to form a toner image on the surface of the image carrier. The toner image formed in this manner is transferred by a transfer unit such as a transfer roller from the surface of the image carrier to a printing material (an output sheet) or an intermediate transfer body. As the image forming apparatus, there is conventionally known a so-called tandem structure in which a plurality of the image carriers corresponding to respective toner colors are arranged in a direction in which the printing material or the intermediate transfer body is conveyed.
In the image forming apparatus in which the plurality of image carriers are arranged, there is a case that the development contrast of a toner image transferred by a succeeding image carrier differs between a portion of the printing material (or the intermediate transfer body) to which a toner image is transferred by a preceding image carrier and a portion of the printing material (or the intermediate transfer body) to which a toner image is not transferred by the preceding image carrier. As a result, there may occur a phenomenon called “a transfer memory” in which density unevenness appears in one image. This “transfer memory” will be described in detail.
For example, in an image forming process using negative toner (negatively charged toner) and employing a reverse developing method, a charging unit (a charging roller) uniformly charges the surface of a photosensitive drum to a dark part potential VD. Then an exposing unit emits image exposure light to the photosensitive drum according to an image density to change the potential of part of the surface of the photosensitive drum to a bright part potential VL, whereby contrast between the dark part potential VD and the bright part potential VL forms an electrostatic latent image. A developing unit applies a developing bias Vdc to the formed electrostatic latent image. A development contrast which is a development potential difference between the developing bias Vdc and the bright part potential VL forms a toner image on a portion having the bright part potential VL of the surface of the photosensitive drum.
The toner image is transferred from the photosensitive drum to the printing material or the intermediate transfer body by applying a positive (positive-polarity) transfer bias Vt to the transfer unit (the transfer roller). On this occasion, in a case where toner transferred from the preceding photosensitive drum is already present in the printing material or the intermediate transfer body, the toner functions as a resistor, and a transfer current does not easily pass through from the transfer roller to the succeeding photosensitive drum. Accordingly, the absolute value of the potential of the succeeding photosensitive drum at a position where the transferred toner exists is higher than that of the succeeding photosensitive drum at another position. In a case where a potential difference is too large before charging by the charging roller, the charging roller cannot remove the potential difference completely, the potential does not become uniform after charging by the charging roller, the absolute value of the potential becomes higher than the dark part potential VD, and accordingly, even at the time of forming an electrostatic latent image by using the exposing unit, the absolute value of the potential becomes higher than the bright part potential VL. A development potential difference between the high absolute value of the potential and the developing bias Vdc at the time of forming the toner image by using the developing unit becomes smaller than an original development potential difference between the developing bias Vdc and the bright part potential VL. Accordingly, the amount of toner and the density become small as compared with another portion. As a result, the effect of toner transferred from the preceding photosensitive drum appears as “the transfer memory” in the image formed after the succeeding photosensitive drum rotates one round. On this occasion, as the amount of toner which has been transferred to the printing material or the intermediate transfer body becomes larger, “the transfer memory” is more likely to occur.
With respect to this “transfer memory,” there is known a method for reducing “the transfer memory” by emitting uniform exposure light to the photosensitive drum to perform “pre-exposure” for making uniform the potential of the photosensitive drum before the charging unit (the charging roller) charges the photosensitive drum to the dark part potential VD.
Further, Japanese Patent Laid-Open No. 2005-250387 discloses a method for changing a voltage used for charging a photosensitive drum to a voltage value for reducing “the transfer memory” in a case where it is determined that an image having a large amount of toner and a high density is included.
Reduction of “the transfer memory” with “pre-exposure” adversely affects the life of the photosensitive drum, and accordingly, it is not preferable to perform “pre-exposure” during processing for printing all images. Therefore, only in the case of printing an image including an image having a large amount of toner and a high density, it is requested to perform “pre-exposure.” Japanese Patent Laid-Open No. 2005-250387 discloses a technique of reducing “the transfer memory” by determining whether or not a high-density image is included. However, in a case where there is no image processing hardware for detecting a high-density image, it is impossible to determine whether or not a high-density image is included. Further, in a case where there is separately mounted software or hardware for detecting a high-density image, the speed of printing processing is lowered.